DRASTIC-FL: a dynamic flood-driven groundwater vulnerability index

Flood events—whether riverine, coastal, groundwater, or pluvial—can rapidly alter groundwater quantity and quality through accelerated recharge and increased hydraulic connectivity with near-surface pollution sources. Climate change has intensified and is projected to further increase the frequency, depth, and spatial extent of flood events, amplifying existing groundwater vulnerabilities. Despite this, systematic assessment of aquifer vulnerability to flooding remains underexplored, in part because standard vulnerability indices such as DRASTIC produce a single static score that does not represent how parameters shift over the course of a storm or a season. This paper presents DRASTIC-FL, a time-variant extension of the DRASTIC framework that produces a dynamic, flood-driven vulnerability index, DFL( t ), expressed as intervals rather than single point values. The DFL( t ) captures the combined effect of DRASTIC parameters through structured expert judgment expressed as bounded ranges and propagates those ranges using interval arithmetic rather than collapsing them into a single point estimate. The framework is applied to two contrasting settings — the UK Chalk aquifer and a San Francisco Bay Area site (Oakland, California) — and shows a gradual, antecedent-moisture-driven shift in vulnerability between drought and post-storm conditions. This indicates that assessments anchored in independent single storm events may understate the risk of whiplash conditions. The DFL( t ) can be used to evaluate the need for avoidance, resilience, mitigation, and remediation, thereby supporting cost-effective, risk-based decision-making in an increasingly flood-prone future.